It is desirable to provide filter media in certain streams of air to remove particulates from streams of air because such particulates can cause damage to equipment, for example, HVAC equipment. It is further desirable to provide filtering media that is efficient, has a long operating life, and has consistent operating characteristics.
It is common in the art to use a filtering media that may be made of any of a variety of materials, including, for example, media made primarily of non-woven cotton-polyester fiber blends. However, non-woven cotton-polyester fiber blends have inherent variability in performance, primarily due to the inconsistency of the fiber diameter in these blends. Additionally, more inconsistency is introduced by additives such as tackifiers or binder agents, which are often used with cotton-polyester fiber blends to increase filter efficiency, because these additives tend to build up in some areas.
One means to combat this inconsistency is to use as a filtering media non-woven blends based on polyester or polyethylene virgin fibers. The increased consistency when using blends based on polyester or polyethylene virgin fibers results primarily from the ability to manufacture these fibers with relatively low variation in diameter from fiber to fiber, as well as from the fact that these blends are effective without additional tackifiers or binder agents. However, filtering media composed primarily of polyester or polyethylene virgin fibers tends to be stiffer than filtering media using cotton-polyester fiber blends. This stiffness can create a problem in filters utilizing a cardboard frame.
It is common in the art to use cardboard frames to provide structural support for the filtering media. Such frames often form a perimeter around the filtering media and include a plurality of support trusses. Angled trusses transfer tension from the top and bottom sides of the frame perimeter to the left and right sides of the frame perimeter. These angled trusses also help resist torsion of the frame. During assembly, frames may be stretched to accept the filtering media. If the media is too stiff it may overcome the yield point of the frame and cause deformation of the frame. Such deformation may prevent the frame from returning to its original shape. This inability of the frame to return to its original shape is especially prevalent when materials such as cardboard or beverage board are used to construct the frame. The deformation causes tension in the truss members, which in turn pulls the left and right sides of the frame inwardly. The deformation of the frame may cause bowing. This bowing is problematic, especially where filters are placed side by side in operation, because bowing of the frame creates gaps through which air can flow unfiltered.
One solution to the problem of frame deformation is to use a more rigid frame. Plastics or metals, for example, may be used to construct the frame and thereby increase its rigidity. However, these materials are prohibitively expensive in comparison to cardboard and other materials.
Thus, it would be highly desirable to use a stiffened filtering media made of, for example, polyester or polyethylene virgin fibers, contained within a cardboard, or similar, frame that has increased resistance to deformation.